air in hot water pipework - vented system

[typhooon]

Have just plumbed in a 160lt indirect tank in my loft with CWS on a platform above. Base of which is in line with top of tank. The horizontal pipe run to vent and distribution is about 750mm. The vent runs at an angle 45 degrees -ish to above the CWS.
On trying out the system I'm finding that air is being sucked in through the vent and into the distribution pipework to the bathroom on the ground floor (2 storey). Any ideas as to how I can eliminate this.

 

[doitall]

Raise the cold storage tank and increase the cold feed to the cylinder.

 

[typhooon]

The tank is already up against the rafters. What do you mean about increase cold feed. I have used 22mm. It seems to me that it is something to do with the nature of the flow at the junction of distribution and vent. I'm wondering if I changed the vent to run up vertically then horizontally instead of diagonally.

 

[doitall]

No it's to do with putting the cylinder in the loft.

Increasing the cold feed to 28mm and preferably 35mm will help prevent the suction on the vent.

 

[typhooon]

ok thanks for your advice doitall